When an outbreak of an infectious disease occurs – thinks SARS or the newly identified MERS virus – human carriers who show no outward signs of the disease are often to blame for the spread of infection.

Case in point: Europeans who traveled to the Middle East and contracted MERS caused the first-known cases in Europe after returning to their home countries. People with MERS don’t begin to show symptoms for about nine days, according to the latest estimates from the WHO. Because of its asymptomatic incubation period, MERS now has “pandemic potential,” according to WHO experts.

Incubated diseases that are passed from human to human can even devastate entire families. In one Boston family, three generations of individuals are infected with tuberculosis – 15 people in all – though most don’t show any symptoms, meaning they were spreading the disease without realizing it before Boston health officials diagnosed them with TB.

Click through to find out which other diseases can be silent, asymptomatic killers.

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